Automotive engine cooling systems contain a variety of metals and metal alloys such as copper, solder, brass, steel, cast iron, aluminum and magnesium. The vulnerability of such metals to corrosive attack is high due to the presence of corrosive liquids and various ions as well as the high temperatures, pressures and flow rates characteristic of engine cooling systems. The presence of corrosion products within a cooling system can also interfere with heat transfer from the engine combustion chambers which may subsequently cause engine overheating and engine component failure.
Corrosion inhibitors are commonly added to engine coolants, e.g., silicates are added to provide aluminum protection, nitrites are added for cast iron protection and azoles may be added for copper and brass corrosion protection and to assist in the protection of iron and steel. All corrosion inhibitors employed in automotive antifreeze/coolant formulations are gradually depleted by use. The life expectancy of most coolants is about one to three years due to the progressive depletion of the corrosion inhibitor component(s). Carboxylic acids in the form of their salts have been incorporated into engine coolants to provide a greater degree of corrosion protection than other known types of corrosion inhibitors. Carboxylates are superior due to their slower depletion rates compared with other corrosion inhibitors. The life expectancy of carboxylate-containing coolants are typically five years or more.
For proper coolant maintenance, the engine operator should routinely monitor coolant levels to determine that the coolant is providing suitable boil and freeze point protection. To maintain adequate levels of corrosion inhibitor, it is also essential that the engine operator continually monitor corrosion inhibitor level as well as water content, coolant level, and visual appearance of the coolant.
In order that an engine coolant provide adequate corrosion protection, it is necessary that the coolant's corrosion inhibitors be present at a certain minimum level. When the concentration drops below this minimum, engine components may be damaged due to corrosion, so the inhibitors should be replenished as needed. Replacement of the entire coolant may be required when severe deterioration or contamination occurs. Inhibitor levels can often drop below the desired level when the engine coolant is inadvertently or intentionally topped off with either water or with another coolant product. Without appropriate tests, it is often difficult or impossible to determine that the required minimum corrosion inhibition is present. Of course a coolant can be sent to the appropriate laboratory for analysis to determine inhibitor level but this is costly and time consuming. In order that the engine operator can take timely corrective action, it is important that a quick determination can be made as to the level of protection afforded by the coolant.
There are field test available but typically they have limited applicability or are difficult to use. For example, nitrite test strips can be used to alert the user to low nitrite levels. Nitrite is required by some coolants in order to provide cast iron cylinder liner protection. These test strips have limited applicability in that they give no indication about protection of all other cooling system metals, such as aluminum, solder and copper. Carboxylates as well as phosphate, borate and tolyltriazole are inhibitors that provide protection for these metals. There are no reliable field tests for phosphate, borate and triazole inhibitors.
Some carboxylate-based coolants (also referred to as OAT coolants) provide a broad spectrum of corrosion protection for an array of metals including iron, steel, aluminum, solder and copper. There are field tests to determine alkyl carboxylate levels but these tests are difficult to apply, involving several steps and coolant manipulations. Because of the difficulties involved, the end user may be hesitant to perform the test and so may let his system go unmonitored. A more dangerous situation may develop if the field test is used incorrectly suggesting the wrong course of action.
Field tests for alkyl carboxylate-based coolants are described in U.S. Pat. Nos. 5,952,233 and 5,744,365. These patents describe a process where a predetermined amount of alkyl carboxylate-based coolant is treated with a predetermined quantity of aluminum cation solution. An aluminum carboxylate soap will precipitate from solution. The resulting mixture is then filtered and the filtrate is tested for the presence of aluminum using an Indicator solution under conditions that must be adjusted so that the indicator will react with aluminum cation. A problem with this method is that if interfering ions are present in the coolant, these ions must be removed by additional chemical treatment prior to testing for aluminum ion. Interfering ions are those ions which will also react with the indicator to give a color change and thus confuse interpretation of the aluminum analysis if they are not removed in advance. If the presence of aluminum is detected in the filtrate, then there was insufficient alkyl carboxylate in the initial coolant to provide adequate protection. Conversely, if aluminum is not detected in the filtrate, then there was sufficient carboxylate for adequate protection and the coolant passes.
A simplified summary of the process taught in U.S. Pat. Nos. 5,952,233 and 5,744,365 for carboxylate determination is depicted by the following reactions:EHA+Al+3→Al(EHA)2 soap+Excess Al+3 Al+3+color indicator→Color change
In this scheme EHA refers to the carboxylate component in a coolant sample, specifically, ethylhexanoate. In this method, the end user must first react coolant with aluminum and then test the resultant reaction mixture for excess aluminum. Another problem with this method, therefore, is that at best this is a two step process and in reality several steps including pH adjustment and filtration are required in order to develop the aluminum indicator colored complex. In addition, as previously mentioned, if there are any known interfering ions present in the coolant, they must be remove before the indicator can be added, thus adding yet another step. If unknown interfering ions are present, no corrective action can be taken and erroneous results are obtained.
In view of available tests, there is a need for a simple-to-use field test that overcomes the deficiencies of known methods for determining a coolant's ability to protect a broad array of cooling system metals.